The after mentioned patent document 1 discloses a vertical double disc surface grinding machine in which a pair of vertically opposing grinding wheels is rotated by grinding wheel rotation drive motors and is vertically moved by grinding wheel vertical drive motors, so that surface grindings of upper and lower ground surfaces of a work-piece are carried out simultaneously.
In the case of the grinder of this kind, when a large number of work-pieces are ground continuously, first, the upper and lower grinding wheels are positioned at initial positions which are away from the ground surfaces of the work-piece by a predetermined distance. The initial positions normally are set in the following manner with respect to the first work-piece.
First, the first work-piece is inserted between the upper and lower grinding wheels, and the lower grinding wheel is vertically elevated or lifted by a manual sighting or observation until the lower grinding wheel comes into contact with a lower surface of the work-piece. Next, the upper grinding wheel is lowered by the manual sighting or observation until the upper grinding wheel comes into contact with an upper surface of the work-piece.
Both the grinding wheels are moved away from the positions where they come into contact with the work-piece by the predetermined distance, and these positions are stored or memorized in a controller of the grinder as the initial positions.
In the subsequent grinding operation, when each work-piece is inserted between the upper and lower grinding wheels, the grinding wheels are on standby at the initial positions, and after the work-piece is inserted, the grinding wheels are vertically moved from the initial positions as a starting point, and the grinding operation is carried out.
Patent Document 1: Japanese Unexamined Patent Publication No. 2002-307270